A man travels from Pune to Mumbai at 78 km/hr and returns at 91 km/hr. Find its average speed 

Correct option is A

Given:
Speed 1 = 78 km/hr
Speed 2 = 91 km/hr
Formula Used:
​$$\text{Average Speed (equal distance)} = \frac{2ab}{a + b}$$​​
Solution:
​$$\text{Average speed} = \frac{2 \times 78 \times 91}{78 + 91}\\\ \\= \frac{14196}{169}\\\ \\= 84 \text{ km/hr}$$​​